The present invention relates to a vehicle gearbox, comprising a basic gearbox with an input shaft mounted in a housing, at least one lay shaft driven by the input shaft, a main shaft, pairwise interengaging gears carried by the lay shaft and the maimshaft, of which at least one gear in each pair is freely rotatably mounted on its shaft and is lockable on its shaft by means of an engaging sleeve nonrotatably but axially displaceably disposed on the shaft, said engaging sleeve cooperating with synchronizing means, a shift control, the movement of which in one direction from a neutral position via motion-transmitting elements produces displacement of an engaging sleeve to an engaging position, and the movement of which from the neutral position in another direction produces a displacement of another engaging sleeve to an engagement position, and a high-low range gearbox with an output shaft and lock-out means comprising a blocking element with operating means, which under predetermined operating conditions holds the blocking element in a lock-out position in which it inhibits movement of the shift control.
It is previously known to provide a gearbox of the type described above, having a main shaft coupled to a subsequent range gearbox of planetary type, for example, as described in SE 8306735-5, with lock-out means to prevent misshifting. A lock-out function which prevents shifting in the basic gearbox from the point in time when shifting in the range gearbox has been initiated until this shifting has been completed, i.e. when the shift selector mechanism: has returned to its inactive position, utilizes a sensor which cooperates with the operating cylinder of the range gearbox and senses the position of its piston rod. When the driver preselects a new range (low to high or vice versa), the shifting is initiated by air being supplied to the operating piston of the range gearbox, as soon as the shift lever of the gearbox reaches the neutral position. During the initial movement of the operating cylinder, a valve, under the influence of the sensor, controls the compressed air to an operating cylinder connected to the blocking element and the operating cylinder moves the blocking element to a position in which it locks the controller shaft in the neutral. the blocking element to a position in which it locks the controller shaft in the neutral position. When the shifting of the range gear has been completed, the operating cylinder of the blocking element is evacuated and spring means cooperating with the blocking element move the blocking element out of its blocking engagement, so that a new gear can be engaged in the basic gearbox.
Another preventer function which prevents shifting from high range to low range at speeds above a predetermined speed, uses a speed sensor which controls a magnetic valve in the compressed air supply line to the operating cylinder of the range gearbox. When the vehicle speed exceeds the predetermined speed limit, the magnetic valve closes and prevents compressed air from being supplied to the operating cylinder.
In gearboxes for heavy trucks, both with and without a range gearbox, with means of the type described above to prevent misshifting, there are at times so-called servo-synchronizers. Such synchronizers use the rotational energy of the rotating parts of the gearbox itself to reduce the shift lever force required for the synchronizing work. In principle, shifting, after manual initiation of the synchronizing sequence (a slight movement of the shift lever from the neutral position towards a gear-engaged position) may be effected entirely by the servo-system. This, however, involves a number of risks. In a known gearbox with a basic gearbox and a high-low range gearbox, the lever position for fourth gear will be the same as for first gear. When shifting from third to fourth, the low range gear engaged in the range gearbox must first be shifted to high range and third must then be shifted to first in the basic gearbox. Shifting in the range gearbox is normally done by pre-selection with a selector placed on the gear lever of the basic gearbox, and the shifting itself is initiated when the gear lever reaches the neutral position, where it is kept blocked as long as the shift mechanism in the range gearbox is in motion. If, however, the driver forgets to preselect the high range when shifting from third to fourth, the result will instead be, unintentionally, an attempt to engage first gear.
Since the shifting mechanism in the range gearbox is not in movement, the gear lever of the basic gearbox will not be blocked and the driver can move the lever into the position for first gear. With conventional synchronizing devices, due to the great difference in rotational speed when shifting down from third to first, this shifting will be so difficult that the driver will notice that he is attempting to engage the wrong gear. With servo-synchronizers or other effective synchronizing means, for example triple synchronizers, this will not be noticed as clearly and the shifting can be completed, with a risk of serious mechanical damage, for example, exploding clutch leaves, which can result in personal injury.
The purpose of the present invention is to achieve a gearbox of the type described by way of introduction, which is so equipped that the described misshifting, caused by forgetting to shift from low range to high range, cannot be completed.
This is achieved according to the invention by virtue of the fact that the operating means of the blocking element are coordinated with a first sensor disposed to sense the speed of the output shaft of the range gearbox, and a second sensor disposed to sense the engaged range in the range gearbox, and that the operating means holds the blocking element in a lock-out position, which prevents movement of the shift control towards the position for the lowest synchronized gear in the basic gearbox, when said speed exceeds a predetermined speed in the low range of the range gearbox.